[Nanyika]

Jayfar is correct that there was never an elevated line on Washington Avenue, although the surface railroad on the avenue did carry passengers during the 19th century. There was an elevated line on S. Delaware Avenue, which was discontinued in 1939. The elevated line on Front Street above Market was indeed demolished, as TonyTone points out, but the line was merely relocated to the center of the I-95 expressway.

One subway line that was unfortunately dismantled involved the connection between the Broad-Ridge Spur and the Locust Street Subway (now PATCO). I think a resumption of that connection as part of the SEPTA system, especially with a new station at Rittenhouse Square, would help to alleviate the congestion on crosstown buses -- which was getting unbearable before the pandemic.

I don't think that "narrow streets" are a major obstruction to building subways. London also has narrow (and very crooked) streets, but the subway system is generally deep underground. It is ironic that much poorer countries, like Egypt and India, are able to build extensive subway systems, but the United States claims that it can no longer "afford" to significantly aid the cities in capital transit improvements. In reality, the U.S. federal government has enough funds to finance major extensions of the country's transit systems, but spending on the military and wars is its priority.